Radiant heating panels and resistive compositions for the same



United States Patent 3,396 055 RADIANT HEATING PANELS AND RESISTIVE COMPOSITIONS FOR THE SAME Walter A. Hedden, Worthington, and Donald J. Bowers and Burnham W. King, Jr., Columbus, Ohio, assignors, by mesne assignments, to Vitreous Steel Products Company, Cleveland, Ohio, a corporation of Delaware No Drawing. Filed Apr. 16, 1965, Ser. No. 448,888

11 Claims. (Cl. 117227) This invention relates to the manufacture of resistance heating circuits and to compositions which comprise such circuits.

Radiant heating panels include a non-conductive base panel, such as porcelain enamel-coated steel, having a circuit path thereon which comprises a conductive material of a suitable electrical resistance to serve as a resistance heating element. When the circuit path is connected to a power line by suitable leads, the panel generates a uniform, clean radiant heat.

Radiant heating panels have been provided with various circuit materials that exhibit acceptable heating characteristics during use. One such material is aluminum, which is flame sprayed onto a porcelain surface while the aluminum is in the liquid state. A portion of the metal is then removed in order to provide .a zigzag pattern on the porcelain. The removal of the excess metal requires an additional processing step, which increases the cost of such a panel. Furthermore, the thickness of the sprayed aluminum circuit must be about 0.002. inch so that the circuit exhibits optimum resistivities. Such a thin film may be easily removed by inadvertent scraping.

In order to overcome the expense involved in flame spraying a metal on a porcelain panel, attempts have been made to silk-screen conductive coatings onto the porcelain base and then fuse the coating into a coherent mass by heating the panel and its coating to a relatively high temperature. Such temperatures are between 1000 and 1400 F. Since most metals oxidize at these firing temperatures, it is customary to employ noble metals, such as powdered gold and/or powdered silver, as the major constituent for the circuit. The circuit material, therefore, cost-s many dollars per ounce and is not economically feasible for most applications.

Accordingly, it has been the practice to employ either an expensive method for applying an inexpensive material to a porcelain base or an inexpensive method for applying an expensive material to a porcelain base. According to this invention, however, desirable heating characteristics may be achieved by employing both an inexpensive method and an inexpensive material.

Thus, it is a principal object of the present invention to provide a conductive circuit material that may be inexpensively applied to a porcelain sheet and which will strongly adhere to the sheet after firing at high temperatures without deleterious oxidation at those temperatures.

In accordance with the invention, conventional porcelain-enameled steel panels are used as substrates for the electrical resistive heating circuitry of this invention. A metal powder composition having oxidation resistance at a sintering temperature of between 1000 and 1400 F. is employed for the electrical circuit. The conductive Patented Aug. 6, 1968 metals, according to this invention, include major amounts of tin with minor amounts of nickel-silver (a copper base alloy containing 17 to 32% Zinc and 10 to 30% nickel), antimony, and silver. Pulven'zed glass (frit) is added to the metallic mixture primarily to bond the mixture to a porcelain enamel substrate. Conventional enameling frits, such as enameling frits for aluminum or cast iron having a normal maturing temperature of about 1250 F., may be employed.

It has been found that a highly suitable circuit is produced by employing a circuit material having a metallic matrix within the following ranges:

TABLE 1 Material: Parts by weight Tin powder 6070 Nickel-silver powder 1 10-20 Antimony powder 10- 20 Silver paste up to 10 1 Copper 64% zinc 18% nickel 18%.

and replacing equal parts by weight of tin.

Glass frit is added to the metal powder with additions of some 2 to 15 parts by weight per 100' parts of metal powder. Preferably, the glass is added to the metal powder at about 5 parts of glass per 100 parts of metal powder. Although the particular frit selected is not critical, the following typical composition expressed in parts by weight is acceptable: PbO, 42%; SiO 35%; TiO 10%; Na O, 8%; K 0, 2%; U 0, 1%; B 0 2%.

The following specific compositions were employed for circuit material:

TABLE 2 Composition Tin Nickel- Antimony Silver 2 Glass Designation Silver 1 Frit 1 Composition by weight: copper 64%; nickel 18%; zinc 18%. 2 DuPont No. 7713 conductive silver paste having about solids.

Each circuit material composition designated in Table 2 was blended and mixed with a volatile screen oil to give each mixture a paste-like consistency. Each mixture was then applied as thin, narrow strips, Ai-inch wide and 3 /2 inches long, on the enamel surface of 4" x 4" panels by screening the mixture with a rubber squeegee through a 6'0-mesh stainless steel screen. The deposited material was then dried under heat lamps at about 300 F. and then fired by heating in air at temperatures of between 1000 and 1400 F. for 2, 3, and 4 minutes. The temperatures employed subjected the metal powder composition to a liquid phase sintering operation to provide a continuous metal matrix for electric conduction.

As will become apparent, the resistivity of the resulting circuits increased due to partial oxidation of some of the components of the composition, but such oxidation resulted in resistivities within acceptable limits. It is believed that, as the composition approaches and reaches the firing temperature, the silver becomes liquid and coats the tin before the tin is heavily oxidized. However, acceptable circuits have been produced by omitting silver. Heavy oxidation may be prevented by the antimony and the glass frit, since antimony may be preferentially oxidized and the glass frit melts to form a coating, thus protecting the metals that would otherwise be heavily oxidized.

2. A composition for a resistive heating circuit comprising by weight between about 10 to 20 parts nickelsilver, between about 10 to 20 parts antimony, up to 10 parts silver, between about 2 to 15 parts glass frit, and the balance tin.

During the firing of the screened circuit, the porcelain 5 eatm c1rcu1t comenamel base becomes soft and thereby wets and 1s wetted rg'in s g fi g f zg i g g fi 5 parts nickeb by the circuit material to form a strong mechanical bond. g y g b Aft the m thickness was between about 0008 silver, between about 10 to 20 parts antimony, etween g 0 6 h s p about to 10 parts silver, 5 parts glass frrt, and the an me I l0 balance tin.

The ends of the fired str1ps were then coated with silver A composition for resistive heating circuit paint to provide electrical contacts, and these contacts prising by Weight 5 Parts tin, 20 parts i k p il 10 were placed on each strip to leave an effectlve circuit parts antimony 5 parts il d 5 parts glass flit length of 3% inches. The resistances of the strips were 5 A composition fo a resistive h i i i measured by an ohmmeter, and the following values were prising by weight 65 parts tin, 10 parts nickel-silver, obtained: parts antimony, 5 parts silver, and 5 parts glass frit.

TABLE 3 Resistance after Sintering at Temperature Shown Composition I A Designation 1,100 F., 4 min. 1,200 F., 4min. 1,300 F., 4 min. 1,400 F., 2 min. 1,400 I"., 3 mm. 1,400 F., 4 111111.

ohm ohm/sq.* ohm ohm/sol ohm ohm/sq.* ohm ohm/Sq.* ohm ohm/sq.* ohm ohm/sq.*

sq.*=eircuit material M inch X inch and a thickness between about 0.008 and 0.010 inch.

Table 3 indicates that the firing temperature should ceptable resistivities of between about .02 ohm/ sq. and 0.1 ohm/sq, since higher temperatures tend to heavily oxidize the components, and lower temperatures do not adequately sinter the compositions.

It has been found that the L-22 circuit material is the most suitable for commercial applications. A firing temperature of between about 1340 and 1380 F. with a maximum heating time of 1% minutes produces the most consistent results and the most desirable resistance values. The L-22 circuit material was printed on a 4" x 4 porcelain-enameled steel panel by the screening techniques set forth above. The pattern, however, was a zigzag pattern comprising a continuous strip of circuit material fivinch wide and 33 inches long. This pattern was fired at 1380" F. for about 1% minutes. The average room temperature resistance was measured at 7.5 ohms. At an operating temperature of near 300 F., however, there is an expected increase in the resistance of the strip of about At operating temperatures, therefore, the average resistance is about 10.5 ohms, or 0.08 ohm/sq. Using this estimated resistance value, the number of A x A squares required for a given service voltage and desired wattage may be calculated as follows. For a service voltage of 120 volts and a desired wattage of 450, the current is 3.8 amperes (450 watts divided by 120 volts). The calculated resistance of the circuit, therefore, is 31 ohms (120 volts divided by 3.8 amperes). Therefore, the total number of squares required is equal to 31 ohms divided by 0.08 ohm, or 390 squares.

The length for a single path resistance circuit will vary with the circuit width (assuming a substantially uniform thickness of between about 0.008 and 0.010 inch). The circuit width, length, and pattern depend upon the particular design and desired output for the entire heating unit. A closely spaced zigzag pattern, however, produces uniform heat output over the entire panel surface.

The invention is not restricted to the slavish imitation of the details set forth above. Obviously, various changes occur to those skilled in the art without departing from the scope of the invention set forth in the appended claims.

What is claimed is:

1. A composition for a resistive heating circuit comprising by weight between about 10 to 20 parts nickelsilver, between about 10 to 20 parts antimony, between about 2 to 15 parts glass frit, and the balance tin.

6. A composition for a resistive heating circuit comprising by weight 60 parts tin, 10 parts nickel-silver, 20 parts antimony, 10 parts silver, and 5 parts glass frit.

7. A heating panel comprising a porcelain base and a resistive heating circuit mechanically bonded to said base,

r' said circuit having a thickness of between about 0.008 to 0.010 inch and having a continuous conductive metal matrix comprising by Weight between about 10 to 20 parts nickel-silver, said nickel-silver comprising by weight about 64 parts copper, about 18 parts nickel, and about 18 parts zinc; 10 to 20 parts antimony; up to 10 parts silver; between about 2 to 15 parts glass frit; and the balance tin.

8. A heating panel comprising a porcelain base and a resistive heating circuit mechanically bonded to said base, said circuit having a thickness of between about 0.008 to 0.010 inch and having a continuous conductive metal matrix, said circuit comprising by weight about 65 parts tin; about 20 parts nickel-silver, said nickel-silver comprising by weight about 64 parts copper, about 18 parts nickel, and about 18 parts zinc; about 10 parts antimony; about 5 parts silver; and about 5 parts glass frit.

9. A heating panel comprising a porcelain base and a resistive heating circuit mechanically bonded to said base, said circuit having a thickness of between about 0.008 to 0.010 inch and having a continuous conductive metal matrix, said circuit comprising by weight about 65 parts tin; about 10 parts nickel-silver, said nickel-silver comprising by weight about 64 parts copper, about 18 parts nickel, and about 18 parts zinc; about 20 parts antimony; about 5 parts silver; and about 5 parts glass frit.

10. A heating panel comprising a porcelain base and a resistive heating circuit mechanically bonded to said base, said circuit having a thickness of between about 0.008 to 0.010 inch and having a continuous conductive metal matrix, said circuit comprising by Weight about 65 parts tin; about 10 parts nickel-silver, said nickel-silver comprising by Weight about 64 parts copper, about 18 parts nickel, and about 18 parts zinc; about 20 parts antimony; about 10 parts silver; and about 5 parts glass frit.

11. A process for manufacturing a heating panel comprising a porcelain base and a resistive heating circuit mechanically bonded to said base, comprising the steps of providing a porcelain base, depositing a resistive heating circuit having a thickness of between about 0.008 to 0.010 inch, the material of said circuit comprising a paste-like 5 6 mixture of a major amount of up to about 70 parts by References Cited weight powdered tin blended with a minor amount of UNITED STATES PATENTS a mixture conslstlng essentially of antimony, mckel-sllver, silver, glass frit, and a volatile screen oil, drying the Neleidham 117*212 deposited material, and then heating the material at a 5 9 Dy 6 et a1 117-230; temperature of between about 1340 and 1380 F. and FOREIGN PATENTS for a time which is sufficient to form a continuous metal 248,945 3/1948 Switzerlan matrix but which is insuflicient to cause substantial oxidation of said metal matrix. WILLIAM L. JARVIS, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3,396 ,055 August 6 1968 Walter A. Hedden et a1.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2 TABLE 1 line 4 after "up to 1O insert and replacing equal parts by weight of tin same column 2, line 21 cancel "and replacing equal parts by weight of tin.

Signed and sealed this 10th day of February 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr.

Attesting Officer Commissioner of Patents WILLIAM E. SCHUYLER, JR. 

7. A HEATING PANEL COMPRISING A PORCELAIN BASE AND A RESISTIVE HEATING CIRCUIT MECHANICALLY BONDED TO SAID BASE, SAID CIRCUIT HAVING A THICKNESS OF BETWEEN ABOUT 0.008 TO 0.010 INCH AND HAVING A CONTINUOUS CONDUCTIVE METAL MATRIX COMPRISING BY WEIGHT BETWEEN ABOUT 10 TO 20 PARTS NICKEL-SILVER, SAID NICKEL-SILVER COMPRISING BY WEIGHT ABOUT 64 PARTS COOPER, ABOUT 18 PARTS NICKLE, AND ABOUT 18 PARTS ZINC; 10 TO 20 PARTS ANTIMONY; UP TO 10 PARTS SILVER; BETWEEN ABOUT 2 TO 15 PARTS GLASS FRIT; AND THE BALANCE TIN. 